Paving materials such as asphaltic concretes that are used for roadways, parking areas, walkways and other traffic surfaces have been the subjects of various efforts to improve their properties. Some of these efforts have involved the addition of polymers, including plastics, in attempts to improve the flexibility, strength and life of the paving material. Such efforts have proved either ineffective or too costly.
The increasing need to dispose of or find new uses for previously used or recycled plastics and waste plastics has given incentives to efforts to introduce plastics from waste sources into building or paving materials, either to facilitate their disposal where it is hoped that their introduction does not degrade building or paving material and does not increase its cost, or where it is hoped that their introduction will provide a cost effective improvement in the properties of the building or paving materials. Work has been done to utilize low density plastic and films of selected and graded recycled plastic materials as an additive to the asphaltic binder component of asphaltic concrete paving material in an effort to improve the flexibility and reduce the propensity of the paving material to crack. This effort requires that the recycling task to collect suitable plastic material be selective, or that the material be specifically sorted from a general mixture of recycled plastic material. Such recycled plastic material has a cost that is significantly greater than that of the general ungraded or unsorted recycled plastic material mixture or of the residual recycled plastic material from which more useful grades have been removed.
For example, it has been proposed to melt polystyrene foam with asphalt, to add sand, and to mold the material as a concrete substitute, thereby utilizing the waste plastic. Further, it has been proposed to add waste polyethylene to asphalt for road construction to increase pavement durability. Decreased deformation resistance and increased hardness and ductility have been reported by adding other plastic waste in amounts of, for example, eight percent to paving compounds containing aggregate, where the plastic waste includes specific plastics made of specific combinations of low density polyethylene, cyclophane, cellophane, polypropylene, and polyvinyl dichloride. Fiber reinforced plastics and chopped glass have been proposed for addition to add to asphalt to improve wear resistance and water permeability.
Proposals to use specific waste plastics as additives to asphalt mixes have had the disadvantage of requiring specific collection of the individual material or the sorting of the desired material from the generally collected plastic waste. Such efforts calling for specific plastics are therefor costly. Furthermore, such efforts do little to solve the problem of utilization of vast unsorted, unsortable or unclassified bulk mixtures of plastic waste.
Waste plastics are found in several forms. In one form, bulk masses of particular identified plastic materials are produced as waste in the plastics industry. In other forms, plastics are found in the form of discarded articles and containers. Some such plastics, particularly plastic bags and plastic bottles, are collected in recycling activities. Recycled plastic bottles are classified according to a nationally recognized identification system known as the Plastic Container Code System (PCCS) into seven classes that are being identified by markings on the bottles. These classes are: class 1, polyethylene terephthalate (PETE), class 2 high density polyethylene (HDPE), class 3, vinyl and polyvinyl chloride or PVC (V), class 4, low density polyethylene (LDPE), class 5, polypropylene (PP), class 6, polystyrene (PS) and class 7, all other resins and layered multi-material. For convenience, these classes are used below to identify waste plastics that are also in a form other than that of bottles for which the classes were specifically established.
Recycled plastics of types corresponding to PCCS classes 1 and 2, and sometimes classes 4, 5 and 6, whether in the form of used containers or other forms made of the materials, have been sorted from the general mass of recycled material or separately collected, all at increased cost. Bulk mixtures of recycled plastics from more than one of the PCCS classes, particularly materials from class 7 and from class 3 when mixed with material from other classes, generally have been regarded as lacking utility and are accordingly routed to landfills. Such materials have lacked an alternative use or manner of disposition.
The employment of plastics in asphalt mixes has presented various problems. Many of the plastic additives have lacked an ability to bond to or combine with the asphalt binders of the mix. Chemical treatments have been proposed, but such treatments have been ineffective, add to the cost, and introduce additional noxious and toxic substances into the process, aggravating the waste disposal problems.
Accordingly, there remains a need for a low cost manner of enhancing the properties of paving material and there remains a need for a use of residual plastic waste, particularly unclassified or unseparated materials or materials of mixed classes.